This invention relates to testing and measuring apparatus and more particularly to such apparatus for inspecting metallic plating on electrical interconnections. The invention in an exemplary use is advantageously applicable to the testing of interconnection surfaces of printed wiring boards and it is in this context that an illustrative embodiment of the invention will be described.
As is known, printed wiring boards, in addition to having a pattern of electrical conductors formed thereon, also are provided with printed conductor interconnection fingers. These fingers are arranged on a wiring board in a manner to provide electrical continuity between the conductor pattern and external circuitry when inserted into a specific connector means. In order to ensure a reliable electrical contact with the connector means for the life of the wiring board, a thin gold plating is added to the metallic contacting surface of the fingers. The necessary reliability is achieved, however, only if the gold plating itself presents an even, uncontaminated and continuous surface. Although gold is not vulnerable to the formation of oxides or tarnish films, these products may originate from the substrate metal when pores in the plating expose the underlying metal surface. These corrosion products are frequently able to insulate a contact surface to a degree which renders a reliable electrical contact impossible. It accordingly becomes necessary in the fabrication of printed wiring board interconnections and in other metallic plating applications to examine for, and to locate, any pores in the surface of a plating which extend to the underlying substrate and for films on the surface. In the past, the plated surface, by one method, was visually examined for porosity by superimposing an electrolytic blotter on the surface, discoloration at the pore points then revealing their presence. This method obviously has the shortcoming of being largely subjective with considerable possibility for error. Other visual methods are open to the same possibilities for mistake.
It is, accordingly, one object of this invention to provide apparatus for ensuring the detection of pores in a metallic plating. Another object is to provide such apparatus which will also generate data regarding the number, size, and location of the pores detected.
A further object of this invention is to provide apparatus which in addition to detecting pores in a metallic plating, will advantageously also detect the presence of nonconductive surface films on such plating.